Horizontal germanium gate all around devices are typically made on Ge single wires. These Ge single wires may for example be made starting from Ge on insulator substrate followed by substrate undercutting or starting from Ge on Si followed by substrate undercutting. Using such a method it is not possible to obtain a semiconductor device including a plurality of vertically-stacked horizontal Ge nanowires.
In order to fabricate vertically stacked horizontal Ge gate all around devices (GAA) from an alternating stack of Ge/SiGe, etching of SiGe selective to relaxed or strained Ge may be used.
At the moment, there is no conventional solution, dry or wet, to release Ge wires from a SiGe/Ge multistack layer with a good selectivity of SiGe versus Ge. As a result, sacrificial Ge has to be taken into account, which subsequently puts limitations on its use as densely stacked Ge wires in future technology nodes.
In alternate techniques, a wet etch may be used to remove the SiGe. The wet etchant may for example be tetramethylammonium hydroxide (TMAH). However, it is difficult to remove SiGe selectively to Ge by wet etching. Selectivity of conventional wet etch approaches is currently limited to about 15:1 at best, which is considered insufficient to provide Ge wires having dimensions compatible with expected density rules for target device nodes.